Did your brother set the torque right on the rear? I rebuilt one and it's not fun. You are probably talking about backlash, which will wear the rear gears out eventually. When you put the new carrier in; you have to shim it and check the torque on the yoke to make sure it's correct. You also need gear marking compound to make sure the gears mesh correctly. It's a long and tedious job. I wouldn't want to do it again, but search YouTube and you will find some vids on how to do it...

 

This is why I'm trying to get him to upgrade to a disk brake unit. I know there is a lot of tricks to get a rear end to work correctly. The truck was like that when he bought it and he hasn't ever poped the diff cover. And he probly needs brakes soon and I know when I tore into my rear end it became pretty costly just to replace the brakes like $500 in parts since everything wasn't taken care of and I wish I would have done this swap